This invention relates generally to signal processing systems and more particularly to beamforming controls for phased array antenna systems.
Phased array antenna systems employ a plurality of individual antennas or subarrays of antennas that are separately excited to cumulatively produce a transmitted electromagnetic wave that is highly directional. The radiated energy from each of the individual antenna elements or subarrays is of a different phase, respectively, so that an equiphase beam front, or the cumulative wave front of electromagnetic energy radiating from all of the antenna elements in the array, travels in a selected direction. The difference in phase or timing between the antenna activating signals determines the direction in which the cumulative beam from all of the individual antenna elements is transmitted. Analysis of the phases of return beams of electromagnetic energy detected by the individual antennas in the array similarly allows determination of the direction from which a return beam arrives.
Beamforming, or the adjustment of the relative phase of the actuating signals for the individual antennas (or subarrays of antennas), can be accomplished by electronically shifting the phases of the actuating signals or by introducing a time delay in the different actuating signals to sequentially excite the antenna elements to generate the desired direction of beam transmission from the antenna. Most present-day phased array radars use modulo 2.pi. antenna beamforming called phase-based beam control. This kind of beamforming limits the radar instantaneous bandwidths to approximately 1-2% of the radar carrier frequency. Nevertheless, this narrowband phase-based beamforming is used in nearly all operational phased array radars today.
Modulo 2.pi. electronic shifting of phases of antenna element actuating signals requires extensive equipment, including switching devices (e.g. PIN diodes) to route the electrical signals through appropriate hardwired circuits to achieve the desired phase changes. Electronic or microwave phase shifters are designed for use at a specific frequency, i.e., the chosen radar carrier frequency, and thus have numerous drawbacks when employed in phased array antenna systems using broadband radiation or wide tunable bandwidths for implementing intrapulse beamforming. For example, most hardwired electronic phase shifters are limited to frequency changes of about 5% of the design frequency. The digital phase control microwave phase shifters also provide only a finite set of phase values; for example, a 6 bit phase shifter generates only 64 possible phase shifts.
Present day phase-based electronically controlled phased array radar antenna systems are relatively large, heavy, complex, and expensive systems. These electronic systems require a large number of microwave components such as phase shifters, power splitters, and waveguides to form the antenna control system. This arrangement results in a system with a narrow tunable bandwidth that is relatively lossy, electromagnetically sensitive, and very hardware intensive. In addition, many phased array antenna systems/radars use mechanical scanning in azimuth, with electronic scanning in height. The mechanical scanning systems are typically large, heavy, and slow.
Ideally, a phased array antenna control system should be light, compact, relatively immune to undesirable electromagnetic radiation, and straight-forward to fabricate, operate, and maintain. Such a system also desirably has a wide antenna tunable bandwidth, and inertialess, motion-free high resolution beam scanning ability with application-dependent slow-to-fast scanning speeds. The wide tunable bandwidth provides the radar with a "frequency hopping" capability that makes it difficult to jam or detect. It is additionally advantageous to have an analog beamforming control system that allows a large number of possible phase shift combinations. Such an analog system is in contrast to digital phase control from microwave phase shifters, which phase control provides a fixed number of possible phase actuation signals. This limited number of possible actuation signals in turn limits the phase resolution achievable with the microwave devices, thus limiting the angular resolution of the scanned antenna beam. Further, in conventional electronically controlled phased arrays, the digital microwave phase shifters are also typically used for correcting phase errors that result due to the other microwave devices in the system. Because of the digital nature of the phase shifters, the phase errors can only be partially cancelled. With the liquid crystal (LC) analog phase control, these phase errors can be completely cancelled.
Optical control systems can be advantageously used to generate control signals for phased array antennas. For example, an optical control system for generating differentially time-delayed optical control systems is presented in the copending applications of N. Riza entitled "Reversible Time Delay Beamforming Optical Architecture for Phased Array Antennas," Ser. No. 07/690,421, filed Apr. 24, 1991, allowed Dec. 18, 1991; and "Time-Multiplexed Phased Array Antenna Beam Switching System," Ser. No. 07/826,501, filed Jan. 27, 1992. Both of these copending applications are assigned to the assignee of the present invention and are incorporated herein by reference.
Liquid crystal devices are advantageously used in such phased array antenna optical control systems to selectively adjust the polarization of light beams used in the signal processing. Large size liquid crystal (LC) arrays have been successfully employed in a number of applications, including flat panel projection displays, high definition television, and aircraft cockpit displays. These LC displays typically use nematic liquid crystals, which have relatively high (0.2) optical birefringence and which are readily controlled by small (e.g., 5 volts) electrical signals. Nematic LCs have been used to make commercial displays having a large area and a large number of pixels (e.g.,&gt;one million pixels) at an acceptably low cost using thin-film transistor (TFT) electrical addressing circuits. The size (number of pixels and area of the array) of a two-dimensional (2-D) LC array is an important consideration in choosing the LC type that will provide the highest performance at an acceptable cost. For example, in a state of the art four-faced phased array radar system currently in production, each of the four faces of the antenna has 4400 elements. Thus, to separately control each antenna element using an optical signal control system with a liquid crystal array requires 4400 switching LC elements per 2-D array. Nematic LC's are readily fabricated in large arrays and a number of effective thin-film transistor-based LC addressing techniques have been developed for driving LC pixels in such an array with 5 V video signals. In addition, nematic LCs have shown as good as 4000:1 on/off ratios. As described in the co-pending application Ser. No. 07/826,501, filed Jan. 27, 1992, cited above, time multiplexing techniques can be efficaciously used to provide a nematic liquid crystal based optical control system that has minimal dead times between respective transmit/receive sequences, and nearly 200 beams/second antenna scanning speeds.
It is accordingly an object of this invention to provide a liquid crystal based electrooptic processor that can generate analog phase-based modulo 2.pi. phased array antenna beam control.
It is a further object of this invention to provide a phase-based antenna controller that is relatively compact, lightweight and has an inertialess beam scanning structure.
Another object of this invention to provide a phase-based antenna controller that can provide an antenna controller that has a wide (i.e., in the GHz range) tunable antenna bandwidth with stable phase-control and an independent, analog, phase-error calibration capability for all the elements in the array.
A further object of the present invention is to provide an optical beam switching technique that has low optical losses, low inter-channel crosstalk, and that is readily fabricated for use with a relatively large (e.g.,&gt;1000) number of phased array antenna elements.